In Situ Construction a Stable Protective Layer in Polymer Electrolyte for Ultralong Lifespan Solid‐State Lithium Metal Batteries

Solid‐state lithium metal batteries (SLMBs) are attracting enormous attention due to their enhanced safety and high theoretical energy density. However, the alkali lithium with high reducibility can react with the solid‐state electrolytes resulting in the inferior cycle lifespan. Herein, inspired by the idea of interface design, the 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethanesulfonyl) imide as an initiator to generate an artificial protective layer in polymer electrolyte is selected. Time‐of‐flight secondary ion mass spectrometry and X‐ray photoelectron spectroscopy reveal the stable solid electrolyte interface (SEI) is in situ formed between the electrolyte/Li interface. Scanning electron microscopy (SEM) images demonstrate that the constructed SEI can promote homogeneous Li deposition. As a result, the Li/Li symmetrical cells enable stable cycle ultralong‐term for over 4500 h. Moreover, the as‐prepared LiFePO4/Li SLMBs exhibit an impressive ultra‐long cycle lifespan over 1300 cycles at 1 C, as well as 1600 cycles at 0.5 C with a capacity retention ratio over 80%. This work offers an effective strategy for the construction of the stable electrolyte/Li interface, paving the way for the rapid development of long lifespan SLMBs. A stable solid electrolyte interface layer with multiple phases of LiF, Li2Sx, and Li3N is successfully in situ formed on the electrolyte/Li surface with the ionic liquid of 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethanesulfonyl) imide as the initiator. Impressively, the LiFePO4/PIA‐SPE/Li solid‐state batteries exhibit admirable cyclic stabilities, and the current findings pave a new direction for fabricating long lifespan solid‐state lithium metal batteries.